Spectral feature control apparatus

ABSTRACT

A spectral feature selection apparatus includes a dispersive optical element arranged to interact with a pulsed light beam; three or more refractive optical elements arranged in a path of the pulsed light beam between the dispersive optical element and a pulsed optical source; and one or more actuation systems, each actuation system associated with a refractive optical element and configured to rotate the associated refractive optical element to thereby adjust a spectral feature of the pulsed light beam. At least one of the actuation systems is a rapid actuation system that includes a rapid actuator configured to rotate its associated refractive optical element about a rotation axis. The rapid actuator includes a rotary stepper motor having a rotation shaft that rotates about a shaft axis that is parallel with the rotation axis of the associated refractive optical element.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.16/524,261, filed Jul. 29, 2019, now allowed, and titled SPECTRALFEATURE CONTROL APPARATUS; which is a divisional of U.S. applicationSer. No. 15/295,280, filed Oct. 17, 2016, and titled “SPECTRAL FEATURECONTROL APPARATUS,” which issued as U.S. Pat. No. 10,416,471 on Sep. 17,2019. These applications are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The disclosed subject matter relates to an apparatus for controlling aspectral feature, such as, for example, bandwidth or wavelength, of alight beam output from an optical source that supplies light to alithography exposure apparatus.

BACKGROUND

In semiconductor lithography (or photolithography), the fabrication ofan integrated circuit (IC) requires a variety of physical and chemicalprocesses performed on a semiconductor (for example, silicon) substrate(which is also referred to as a wafer). A photolithography exposureapparatus or scanner is a machine that applies a desired pattern onto atarget portion of the substrate. The wafer is fixed to a stage so thatthe wafer generally extends along a plane defined by orthogonal X_(L)and Y_(L) directions of the scanner. The wafer is irradiated by a lightbeam, which has a wavelength in the deep ultraviolet (DUV) range. Thelight beam travels along an axial direction, which corresponds with theZ_(L) direction of the scanner. The Z_(L) direction of the scanner isorthogonal to the lateral X_(L)-Y_(L) plane.

An accurate knowledge of spectral features or properties (for example, abandwidth) of a light beam output from an optical source such as a laseris important in many scientific and industrial applications. Forexample, accurate knowledge of the optical source bandwidth is used toenable control of a minimum feature size or critical dimension (CD) indeep ultraviolet (DUV) optical lithography. The critical dimension isthe feature size that is printed on a semiconductor substrate (alsoreferred to as a wafer) and therefore the CD can require fine sizecontrol. In optical lithography, the substrate is irradiated by a lightbeam produced by an optical source. Often, the optical source is a lasersource and the light beam is a laser beam.

SUMMARY

In some general aspects, an optical system is used with a light source.The optical system includes a dispersive optical element; a plurality ofrefractive optical elements between the dispersive optical element andlight source; and actuation systems, each actuation system associatedwith a refractive optical element. At least one of the actuation systemsincludes an actuator configured to rotate the associated refractiveoptical element 360 degrees about a rotation axis, the actuatorcomprising a rotation shaft configured to rotate about a shaft axis, theshaft axis being parallel with the rotation axis of the associatedrefractive optical element and the shaft axis lacking a ground energystate.

In some general aspects, a spectral feature selection apparatus is usedwith a pulsed optical source that produces a pulsed light beam. Thespectral feature selection apparatus includes a dispersive opticalelement arranged to interact with the pulsed light beam; three or morerefractive optical elements arranged in a path of the pulsed light beambetween the dispersive optical element and the pulsed optical source;one or more actuation systems, each actuation system associated with arefractive optical element and configured to rotate the associatedrefractive optical element to thereby adjust a spectral feature of thepulsed light beam; and a control system connected to the one or moreactuation systems. At least one of the actuation systems is a rapidactuation system that includes a rapid actuator configured to rotate itsassociated refractive optical element about a rotation axis. The rapidactuator includes a rotary stepper motor having a rotation shaft thatrotates about a shaft axis that is parallel with the rotation axis ofthe associated refractive optical element. The control system isconfigured to send a signal to the rapid actuator to adjust the rotationshaft of the rotary stepper motor to thereby rotate the associatedrefractive optical element about its rotation axis.

Implementations can include one or more of the following features. Forexample, the rapid actuator can be configured to rotate the refractiveoptical element that is farthest from the dispersive optical element.The pulsed light beam path can lie in an XY plane of the apparatus, andthe rotation shaft of the rotary stepper motor can have an axis that isparallel with a Z axis of the apparatus to thereby rotate the associatedrefractive optical element about its rotation axis, which is parallelwith the Z axis of the apparatus.

The rapid actuation system can also include a secondary actuatorphysically coupled to the associated refractive optical element, thesecondary actuator configured to rotate the associated refractiveoptical element about an axis that lies in the XY plane and also lies inthe plane of a hypotenuse of the associated refractive optical element.

The apparatus can include a position monitor that detects a position ofthe rotation shaft of the rotary stepper motor. The control system canbe connected to the position monitor, and can be configured to receivethe detected position of the rotation shaft and to adjust the rotationshaft if the received detected position is not within an acceptablerange of positions. The position monitor can be an optical rotaryencoder. The rotation shaft can be configured to rotate the refractiveoptical element associated with the rapid actuator about an offset axisthat is offset from the rotation axis.

The shaft axis can be configured to rotate about 360° to thereby rotatethe associated refractive optical element about 360°.

The refractive optical element associated with the rapid actuator can befixedly coupled to the shaft axis.

The control system can include a rapid controller connected to therotary stepper motor, the adjustment to the rotation shaft beingperformed by way of the rapid controller.

The rotation of the refractive optical element associated with the rapidactuator can cause a change in the magnification of the pulsed lightbeam that interacts with the dispersive optical element. The change inmagnification of the pulsed light beam can cause a change in thebandwidth of the pulsed light beam. The range of bandwidth due to therotation of the refractive optical element associated with the rapidactuator can be at least 250 femtometers (fm). The rotation of therefractive optical element associated with the rapid actuator by oneunit of rotation of the rotation shaft can cause the bandwidth of thepulsed light beam to change by an amount that is less than a resolutionof a bandwidth measurement device that measures the bandwidth of thepulsed light beam.

The dispersive optical element can be a diffractive optical elementarranged to interact with a pulsed light beam in a Littrow configurationso that the pulsed light beam diffracted from the diffractive opticalelement travels along the path of the pulsed light beam that is incidenton the diffractive optical element.

The refractive optical elements can be right-angled prisms through whichthe pulsed light beam is transmitted so that the pulsed light beamchanges its optical magnification as it passes through each right-angledprism. The right-angled prism that is farthest from the dispersiveoptical element can have the smallest hypotenuse of the plurality, andeach consecutive right-angled prism closer to the dispersive opticalelement can have a larger or same size hypotenuse than the adjacentright-angled prism that is farther from the dispersive optical element.

In other general aspects, a spectral feature selection apparatusincludes a dispersive optical element arranged to interact with a pulsedlight beam produced by a pulsed optical source. The spectral featureselection apparatus includes a plurality of refractive optical elementsarranged in a path of the pulsed light beam between the dispersiveoptical element and the pulsed optical source; and a plurality ofactuation systems. Each actuation system is associated with a refractiveoptical element and is configured to rotate the associated refractiveoptical element to thereby adjust a spectral feature of the pulsed lightbeam. At least one of the actuation systems includes a rapid actuatorthat includes a rotary motor having a rotational shaft that rotatesabout a shaft axis that is perpendicular to a plane of the apparatus.The refractive optical element associated with the rapid actuator ismounted to the rapid actuator so that the refractive optical element isrotated about an offset axis that is parallel with the shaft axis, isoffset from its center of gravity, and is offset from the shaft axis.

Implementations can include one or more of the following features. Forexample, the rapid actuator can cause the associated refractive opticalelement to both rotate about the offset axis and linearly translate. Therapid actuator can include a lever arm that includes: a first regionthat is mechanically linked to the rotational shaft at the location ofthe shaft axis, and a second region that is offset from the shaft axisalong a direction perpendicular to the shaft axis and lying in the planeof the apparatus so that the second region is not intersected by theshaft axis. The refractive optical element associated with the rapidactuator is mechanically linked to the second region.

The apparatus can also include a control system connected to theplurality of actuation systems and configured to send a signal to eachactuation system. The control system can be configured to send a signalto the rapid actuator to thereby rotate and translate the refractiveoptical element associated with the rapid actuator to adjust a spectralfeature of the pulsed light beam. The refractive optical elementassociated with the rapid actuator can be configured to be rotated aboutthe offset axis by 15 degrees and come to a stable equilibrium positionin less than or equal to about 50 milliseconds.

The spectral feature can be one or more of a bandwidth and a wavelengthof the pulsed light beam. The refractive optical element controlled bythe rapid actuator can lack an anti-reflection coating.

In other general aspects, a spectral feature selection apparatusincludes a diffractive optical element arranged to interact with apulsed light beam in a Littrow configuration, the pulsed light beambeing produced by an optical source. The spectral feature selectionapparatus also includes a set of four or more right-angled prismsthrough which the pulsed light beam is transmitted so that the pulsedlight beam changes its optical magnification as it passes through eachright-angled prism. The pulsed light beam travels along a beam path andhas a transverse extent that crosses the hypotenuse of each prism sothat the transverse extent of the pulsed light beam is contained withineach of the hypotenuses of each prism. The right-angled prism that isclosest to the diffractive optical element has a hypotenuse having thelargest length of the set. Each consecutive right-angled prism fartherfrom the diffractive optical element has a hypotenuse having a lengththat is smaller than or equal to the hypotenuse of the adjacentright-angled prism that is closer to the diffractive optical element.The right-angled prism that is closest to the diffractive opticalelement is arranged with its right angle positioned away from thediffractive optical element. The region between the right-angled prismthat is farthest from the diffractive optical element and thediffractive optical element is void of any reflective optical element.The spectral feature selection apparatus also includes at least twoactuation systems, each actuation system associated with a right-angledprism of the set and configured to rotate the associated right-angledprism relative to the pulsed light beam to thereby adjust a spectralfeature of the pulsed light beam.

Implementations can include one or more of the following features. Forexample, the prism that is farthest from the diffractive optical elementcan be associated with an actuation system and can be movable, and theprism that is the second closest to the diffractive optical element canbe associated with an actuation system and can be movable.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a photolithography system producing apulsed light beam, the photolithography system including a spectralfeature selection apparatus for tuning one or more spectral features ofthe pulsed light beam;

FIG. 2 is a graph of an exemplary optical spectrum of the pulsed lightbeam produced by the photolithography system of FIG. 1 ;

FIGS. 3A, 4A, 5A, 6A, and 7 are block diagrams of exemplary spectralfeature selection apparatuses that can be used in the photolithographysystem of FIG. 1 ;

FIG. 3B is a block diagram showing a beam magnification and a beamrefraction angle through one of the refractive optical elements of thespectral feature selection apparatus of FIG. 3A;

FIG. 4B is a side view of a portion of an exemplary beam expander of thespectral feature selection apparatus of FIG. 4A;

FIG. 4C is a top view of a portion of the exemplary beam expander ofFIG. 4B showing an adjustment to the beam expander;

FIG. 5B is a side view of a portion of an exemplary beam expander of thespectral feature selection apparatus of FIG. 5A;

FIG. 5C is a top view of a portion of the exemplary beam expander ofFIG. 5B showing an adjustment to the beam expander;

FIG. 6B is a side view of a portion of an exemplary beam expander of thespectral feature selection apparatus of FIG. 6A;

FIG. 6C is a top view of a portion of the exemplary beam expander ofFIG. 6A;

FIG. 6D is a side view of a portion of the exemplary beam expander ofFIG. 6B showing an adjustment to the beam expander;

FIG. 8 is a block diagram of an exemplary optical source of thephotolithography system of FIG. 1 ; and

FIG. 9 is a block diagram of an exemplary control system of thephotolithography system of FIG. 1 .

DESCRIPTION

Referring to FIG. 1 , a photolithography system 100 includes anillumination system 150 that produces a pulsed light beam 110 having awavelength that is nominally at a center wavelength and is directed to aphotolithography exposure apparatus or scanner 115. The pulsed lightbeam 110 is used to pattern microelectronic features on a substrate orwafer 120 received in the scanner 115. The illumination system 150includes an optical source 105 that produces the pulsed light beam 110at a pulse repetition rate that is capable of being changed. Theillumination system 150 includes a control system 185 that communicateswith the optical source 105 and other features within the illuminationsystem 150. The illumination system 150 also communicates with thescanner 115 to control the operation of the illumination system 150 andaspects of the pulsed light beam 110.

The light beam 110 is directed through a beam preparation system 112,which can include optical elements that modify aspects of the light beam110. For example, the beam preparation system 112 can include reflectiveand/or refractive optical elements, optical pulse stretchers, andoptical apertures (including automated shutters).

The pulses of the light beam 110 are centered around a wavelength thatis in the deep ultraviolet (DUV) range, for example, with wavelengths of248 nanometers (nm) or 193 nm. The size of the microelectronic featurespatterned on the wafer 120 depends on the wavelength of the pulsed lightbeam 110, with a lower wavelength resulting in a small minimum featuresize or critical dimension. When the wavelength of the pulsed light beam110 is 248 nm or 193 nm, the minimum size of the microelectronicfeatures can be, for example, 50 nm or less. The bandwidth that is usedfor analysis and control of the pulsed light beam 110 can be the actual,instantaneous bandwidth of its optical spectrum 200 (or emissionspectrum), as shown in FIG. 2 . The optical spectrum 200 containsinformation about how the optical energy or power of the light beam 110is distributed over different wavelengths (or frequencies).

The illumination system 150 includes a spectral feature selectionapparatus 130. The spectral feature selection apparatus 130 is placed ata first end of the optical source 105 to interact with a light beam 110Aproduced by the optical source 105. The light beam 110A is a beamproduced at one end of the resonators within the optical source 105 andcan be a seed beam produced by a master oscillator, as discussed below.The spectral feature selection apparatus 130 is configured to finelytune the spectral properties of the pulsed light beam 110 by tuning oradjusting one or more spectral features (such as the bandwidth orwavelength) of the pulsed light beam 110A.

Referring to FIG. 3A, the spectral feature selection apparatus 130includes a set of optical features or components 300, 305, 310, 315, 320arranged to optically interact with the pulsed light beam 110A and acontrol module 350 that includes electronics in the form of anycombination of firmware and software. The control module 350 isconnected to one or more actuation systems 300A, 305A, 310A, 315A, 320Aphysically coupled to respective optical components 300, 305, 310, 315,320. The optical components of the apparatus 130 include a dispersiveoptical element 300, which can be a grating, and a beam expander 301made of a set of refractive optical elements 305, 310, 315, 320, whichcan be prisms. The grating 300 can be a reflective grating that isdesigned to disperse and reflect the light beam 110A; accordingly, thegrating 300 is made of a material that is suitable for interacting witha pulsed light beam 110A having a wavelength in the DUV range. Each ofthe prisms 305, 310, 315, 320 is a transmissive prism that acts todisperse and redirect the light beam 110A as it passes through the bodyof the prism. Each of the prisms can be made of a material (such as, forexample, calcium fluoride) that permits the transmission of thewavelength of the light beam 110A.

The prism 320 is positioned farthest from the grating 300 while theprism 305 is positioned closest to the grating 300. The pulsed lightbeam 110A enters the apparatus 130 through an aperture 355, and thentravels through the prism 320, the prism 310, and the prism 305, in thatorder, prior to impinging upon a diffractive surface 302 of the grating300. With each passing of the beam 110A through a consecutive prism 320,315, 310, 305, the light beam 110A is optically magnified and redirected(refracted at an angle) toward the next optical component. The lightbeam 110A is diffracted and reflected from the grating 300 back throughthe prism 305, the prism 310, the prism 315, and the prism 320, in thatorder, prior to passing through the aperture 355 as the light beam 110Aexits the apparatus 130. With each passing through the consecutiveprisms 305, 310, 315, 320 from the grating 300, the light beam 110A isoptically compressed as it travels toward the aperture 355.

Referring to FIG. 3B, the rotation of a prism P (which can be any one ofprisms 305, 310, 315, or 320) of the beam expander 301 changes an angleof incidence at which the light beam 110A impinges upon the entrancesurface H(P) of that rotated prism P. Moreover, two local opticalqualities, namely, an optical magnification OM(P) and a beam refractionangle δ(P), of the light beam 110A through that rotated prism P arefunctions of the angle of incidence of the light beam 110A impingingupon the entrance surface H(P) of that rotated prism P. The opticalmagnification OM(P) of the light beam 110A through the prism P is theratio of a transverse wide Wo(P) of the light beam 110A exiting thatprism P to a transverse width Wi(P) of the light beam 110A entering thatprism P.

A change in the local optical magnification OM(P) of the light beam 110Aat one or more of the prisms P within the beam expander 301 causes anoverall change in the optical magnification OM 365 of the light beam110A through the beam expander 301. The optical magnification OM 365 ofthe light beam 110A through the beam expander 301 is the ratio of thetransverse width Wo of the light beam 110A exiting the beam expander 301to a transverse width Wi of the light beam 110A entering the beamexpander 301.

Additionally, a change in the local beam refraction angle δ(P) throughone or more of the prisms P within the beam expander 301 causes anoverall change in an angle of incidence of 362 of the light beam 110A atthe surface 302 of the grating 300.

The wavelength of the light beam 110A can be adjusted by changing theangle of incidence 362 at which the light beam 110A impinges upon thediffractive surface 302 of the grating 300. The bandwidth of the lightbeam 110A can be adjusted by changing the optical magnification 365 ofthe light beam 110.

As discussed herein, for example, with respect to FIGS. 3A-7 , thespectral feature selection apparatus 130 is redesigned to provide formore rapid adjustment of the bandwidth of the pulsed light beam 110while the light beam 110 is being scanned across the wafer 120 by thescanner 115. The spectral feature selection apparatus 130 can beredesigned with one or more new actuation systems for more effectivelyand more rapidly rotating one or more of the optical components 300,305, 310, 315, 320.

For example, the spectral feature selection apparatus 130 includes a newactuation system 320A for more effectively and more rapidly rotating theprism 320. The new actuation system 320A can be designed in a mannerthat increases the speed with which the prism 320 is rotated.Specifically, the axis of rotation of the prism 320 mounted to the newactuation system 320A is parallel with a rotatable motor shaft 322A ofthe new actuation system 320A. In other implementations, the newactuation system 320A can be designed to include an arm that isphysically linked to the motor shaft 322A at one end and physicallylinked to the prism 320 at the other end to provide additional leveragefor rotating the prism 320. In this way, the optical magnification OM ofthe light beam 110A is made to be more sensitive to rotation of theprism 320.

In some implementations, such as shown in FIG. 7 , the prism 305 isflipped relative to the prior design of the beam expander to provide formore rapid adjustment of the bandwidth. In these cases, the bandwidthchange becomes relatively faster (when compared with prior designs ofthe apparatus 130) with a relatively smaller rotation of the prism 320.The change in optical magnification per unit rotation of the prism 320is increased in the redesigned spectral feature selection apparatus 130when compared with prior spectral feature selection apparatuses.

The apparatus 130 is designed to adjust the wavelength of the light beam110A that is produced within the resonator or resonators of the opticalsource 105 by adjusting an angle 362 of incidence of at which the lightbeam 110A impinges upon the diffractive surface 302 of the grating 300.Specifically, this can be done by rotating one or more of the prisms305, 310, 315, 320 and the grating 300 to thereby adjust the angle ofincidence 362 of the light beam 110A.

Moreover, the bandwidth of the light beam 110A that is produced by theoptical source 105 is adjusted by adjusting the optical magnification OM365 of the light beam 110A. Thus, the bandwidth of the light beam 110Acan be adjusted by rotating one or more of the prisms 305, 310, 315,320, which causes the optical magnification 365 of the light beam 110Ato change.

Because the rotation of a particular prism P causes a change in both thelocal beam refraction angle δ(P) and the local optical magnificationOM(P) at that prism P, the control of wavelength and bandwidth arecoupled in this design.

Additionally, the bandwidth of the light beam 110A is relativelysensitive to the rotation of the prism 320 and relatively insensitive torotation of the prism 305. This is because any change in the localoptical magnification OM(320) of the light beam 110A due to the rotationof the prism 320 is multiplied by the product of the change in theoptical magnification OM(315), OM(310), OM(305), respectively, in theother prisms 315, 310, and 305 because those prisms are between therotated prism 320 and the grating 300, and the light beam 110A musttravel through these other prisms 315, 310, 305 after passing throughthe prism 320. On the other hand, the wavelength of the light beam 110Ais relatively sensitive to the rotation of the prism 305 and relativelyinsensitive to the rotation of the prism 320.

For example, in order to change the wavelength without changing thebandwidth of the light beam 110A, the angle of incidence 362 should bechanged without changing the optical magnification 365. This can beachieved by rotating the prism 305 by a large amount and rotating theprism 320 by a small amount. In order to change the bandwidth withoutchanging the wavelength, the optical magnification 365 should be changedwithout changing the angle of incidence 362, and this can be achieved byrotating the prism 320 by a large amount and rotating the prism 305 by asmall amount.

The control module 350 is connected to one or more actuation systems300A, 305A, 310A, 315A, 320A that are physically coupled to respectiveoptical components 300, 305, 310, 315, 320. Although an actuation systemis shown for each of the optical components it is possible that some ofthe optical components in the apparatus 130 are either kept stationaryor are not physically coupled to an actuation system. For example, insome implementations, the grating 300 can be kept stationary and theprism 315 can be kept stationary and not physically coupled to anactuation system.

Each of the actuation systems 300A, 305A, 310A, 315A, 320A includes anactuator that is connected to its respective optical components. Theadjustment of the optical components causes the adjustment in theparticular spectral features (the wavelength and/or bandwidth) of thelight beam 110A. The control module 350 receives a control signal fromthe control system 185, the control signal including specific commandsto operate or control one or more of the actuation systems. Theactuation systems can be selected and designed to work cooperatively.

Each of the actuators of the actuation systems 300A, 305A, 310A, 315A,320A is a mechanical device for moving or controlling the respectiveoptical component. The actuators receive energy from the module 350, andconvert that energy into some kind of motion imparted to the respectiveoptical component. For example, the actuation systems can be any one offorce devices and rotation stages for rotating one or more of prisms ofa beam expander. The actuation systems can include, for example, motorssuch as stepper motors, valves, pressure-controlled devices,piezoelectric devices, linear motors, hydraulic actuators, voice coils,etc.

The grating 300 can be a high blaze angle Echelle grating, and the lightbeam 110A incident on the grating 300 at any angle of incidence 362 thatsatisfies a grating equation will be reflected (diffracted). The gratingequation provides the relationship between the spectral order of thegrating 300, the diffracted wavelength (the wavelength of the diffractedbeam), the angle of incidence 362 of the light beam 110A onto thegrating 300, the angle of exit of the light beam 110A diffracted off thegrating 300, the vertical divergence of the light beam 110A incidentonto the grating 300, and the groove spacing of the diffractive surfaceof the grating 300. Moreover, if the grating 300 is used such that theangle of incidence 362 of the light beam 110A onto the grating 300 isequal to the angle of exit of the light beam 110A from the grating 300,then the grating 300 and the beam expander (the prisms 305, 310, 315,320) are arranged in a Littrow configuration and the wavelength of thelight beam 110A reflected from the grating 300 is the Littrowwavelength. It can be assumed that the vertical divergence of the lightbeam 110A incident onto the grating 300 is near zero. To reflect thenominal wavelength, the grating 300 is aligned, with respect to thelight beam 110A incident onto the grating 300, so that the nominalwavelength is reflected back through the beam expander (the prisms 305,310, 315, 320) to be amplified in the optical source 105. The Littrowwavelength can then be tuned over the entire gain bandwidth of theresonators within optical source 105 by varying the angle of incidence362 of the light beam 110A onto the grating 300.

Each of the prisms 305, 310, 315, 320 is wide enough along thetransverse direction of the light beam 110A so that the light beam 110Ais contained within the surface at which it passes. Each prism opticallymagnifies the light beam 110A on the path toward the grating 300 fromthe aperture 355, and therefore each prism is successively larger insize from the prism 320 to the prism 305. Thus, the prism 305 is largerthan the prism 310, which is larger than the prism 315, and the prism320 is the smallest prism.

The prism 320 that is the farthest from the grating 300, and is also thesmallest in size, is mounted on the actuation system 320A, andspecifically to the rotation shaft 322A, which causes the prism 320 torotate, and such rotation changes the optical magnification of the lightbeam 110A impinging upon the grating 300 to thereby modify the bandwidthof the light beam 110A output from the apparatus 130. The actuationsystem 320A is designed as a rapid actuation system 320A because itincludes a rotary stepper motor that includes the rotation shaft 322A towhich the prism 320 is fixed. The rotation shaft 322A rotates about itsshaft axis, which is parallel with the rotation axis of the prism 320.Moreover, because the actuation system 320A includes the rotary steppermotor, it lacks any mechanical memory and also lacks an energy groundstate. Each location of the rotation shaft 322A is at the same energy aseach of the other locations of the rotation shaft 322A and the rotationshaft 322A lacks a preferred resting location with a low potentialenergy.

The rotary stepper motor of the system 320A should be fast enough tomove the rotation shaft 322A and therefore the prism 320 rapidly (whichmeans, fast enough to enable the adjustment to the spectral feature ofthe light beam 110A within the required time frame). In someimplementations, the rotary stepper motor of the system 320A isconfigured with an optical rotational encoder to provide feedbackregarding the position of the rotation shaft 322A. Moreover, the rotarystepper motor can be controlled with a motor controller that is a highresolution positional controller and uses a variable-frequency drivecontrol method. In one example, the rotary stepper motor of the system320A is fast enough to move the rotation shaft 322A and the prism 320 15degrees in less than 50 ms and the accuracy of the optical rotationalencoder can be less than 50 microdegrees (for example, 15 degrees in 30ms). One example of a variable-frequency drive control method is vectormotor control, in which stator currents of the motor are controlled byway of two orthogonal components, the magnetic flux of the motor and thetorque. The optical rotational encoder can be an encoder with opticalscanning that incorporates measuring standards of periodic structuresknown as graduations that are applied to a carrier substrate of glass orsteel. In some examples, the optical rotation encoder is made byHEIDENHAIN.

Referring to FIGS. 4A and 4B, in a first implementation, a spectralfeature selection apparatus 430 is designed with a grating 400 and fourprisms 405, 410, 415, 420. The grating 400 and the four prisms 405, 410,415, 420 are configured to interact with the light beam 110A produced bythe optical source 105 after the light beam 110A passes through anaperture 455 of the apparatus 430. The light beam 110A travels along apath in the XY plane of the apparatus 430 from the aperture 455, throughthe prism 420, the prism 415, the prism 410, the prism 405, and then isreflected from the grating 400, and back through the prisms 405, 410,415, 420 before exiting the apparatus through the aperture 455.

The prisms 405, 410, 415, 420 are right-angled prisms through which thepulsed light beam 110A is transmitted so that the pulsed light beam 110Achanges its optical magnification as it passes through each right-angledprism. The right-angled prism 420 that is farthest from the dispersiveoptical element 400 has the smallest hypotenuse of the plurality, andeach consecutive right-angled prism closer to the dispersive opticalelement 400 has a larger or same size hypotenuse than the adjacentright-angled prism that is farther from the dispersive optical element.

For example, the prism 405 that is closest to the grating 400 is alsothe largest in size, for example, its hypotenuse has the largest extentof the four prisms 405, 410, 415, 420. The prism 420 that is farthestfrom the grating 400 is also the smallest in size, for example, itshypotenuse has the smallest extent of the four prisms 405, 410, 415,420. It is possible for adjacent prisms to be the same size. But, eachprism that is closer to the grating 400 should be at least as large orgreater than in size its adjacent prism because the light beam 110A isoptically magnified as travels through the prism 420, the prism 415, theprism 410, and the prism 405, and thus the transverse extent of thelight beam 110A enlarges as the light beam 110A gets closer to thegrating 400. The transverse extent of the light beam 110A is the extentalong the plane that is perpendicular to the propagation direction ofthe light beam 110A. And, the propagation direction of the light beam110A is in the XY plane of the apparatus 430.

The prism 405 is physically coupled to an actuation system 405A thatrotates the prism 405 about an axis that is parallel with the Z axis ofthe apparatus 430, the prism 410 is physically coupled to an actuationsystem 410A that rotates the prism 410 about an axis that is parallelwith the Z axis, and the prism 420 is physically coupled to a rapidactuation system 420A. The rapid actuation system 420A is configured torotate the prism 405 about an axis that is parallel with the Z axis ofthe apparatus 430.

The rapid actuation system 420A includes a rotary stepper motor 421Athat has a rotation shaft 422A and rotation plate 423A fixed to therotation shaft 422A. The rotation shaft 422A and therefore the rotationplate 423A rotate about a shaft axis AR that is parallel with a centerof mass (that corresponds to a rotation axis AP) of the prism 420 and isalso parallel with the Z axis of the apparatus 430. Although notnecessary, the shaft axis AR of the prism 420 can correspond with oralign with the center of mass (the rotation axis AP) of the prism 420along the XY plane. In some implementations, the center of mass (orrotation axis AP) of the prism 420 is offset from the shaft axis ARalong the XY plane. By offsetting the shaft axis AR from the prism 420center of mass, the position of the light beam 110A can be adjusted tobe at a particular position on the surface of the grating 400 wheneverthe prism 420 is rotated.

By mounting the prism 420 to the rotation plate 423A, the prism 420 isdirectly rotated about its rotation axis AP as the shaft 422A androtation plate 423A are rotated about their shaft axis AR. In this way,rapid rotation or control of the prism 420 is enabled when compared witha system that uses a linear stepper motor having a linearly translatableshaft (that is converted into a rotational motion using a flexure).Because a rotational step of the shaft 422A (and plate 423A) directlycorrelates to a rotational step of the prism 420 (without the impartingany a linear motion), the rotary stepper motor 421A is able to rotatethe prism 420 at a speed that enables more rapid adjustment of spectralfeatures (such as the bandwidth) of the light beam 110A and thereforethe light beam 110. The rotary design of the stepper motor 421A impartsa purely rotational motion to the prism 420, which is mounted withoutthe use of any linear motion or flexure motion that are found on prioractuators for the prism 420. Moreover, the use of a rotary shaft 422Aenables the prism 420 to be rotated about a full 360°, unlike the prioractuator that used a linear stepper motor plus a flexure design (inwhich the prism 420 could only be rotated about the angle determinedfrom the flexure). In some implementations, in order to achieve a tuningof the bandwidth of the light beam 110A in an acceptable range, theprism 420 is capable of being rotated by 15 degrees. The prism 420 canbe rotated by larger than 15 degrees though it is not necessary with thecurrent bandwidth range requirements.

In some implementations, the stepper motor 421A can by a direct drivestepper motor. A direct drive stepper motor is a conventionalelectromagnetic motor that uses a built-in step motor functionality forposition control. In other implementations in which a higher resolutionin motion may be needed, the stepper motor 421A can use a piezoelectricmotor technology.

The stepper motor 421A can be a rotary stage that is controlled with amotor controller using a variable-frequency drive control method toprovide the rapid rotation of the prism 420.

As discussed above, the advantage of using a rotary stepper motor 421Ais to obtain more rapid rotation of the prism 420 because the rotationaxis AP of the prism 420 is parallel with the rotational shaft 422A andalso the shaft axis AR. Thus, for every unit rotation of the shaft 422A,the prism 420 rotates by an incremental unit and the prism 420 rotatesas fast as the rotational shaft 422A can rotate. In someimplementations, in order to increase the stability of thisconfiguration, and increase the stability of the prism 420, the rapidactuation system 420A includes a position monitor 424A that isconfigured to detect a position of the rotational shaft 422A of therotary stepper motor 421A. The error between the measured position ofthe rotational shaft 422A and the expected or target position of therotational shaft 422A correlates directly with the error in the positionof the prism 420 and thus, this measurement can be used to determine therotational error of the prism 420 (that is, the difference betweenactual rotation and commanded rotation) and to correct for this errorduring operation.

The control module 350 is connected to the position monitor 424A toreceive the value of the position of the rotational shaft 422A and thecontrol module 350 is also able to access a stored or current value ofthe commanded position of the rotational shaft 422A so that the controlmodule 350 can perform the calculation to determine the differencebetween the measured value of the position and the commanded position ofthe rotational shaft 422A and also determine how to adjust therotational shaft 422A to reduce this error. For example, the controlmodule 350 can determine a size of rotation as well as a direction ofrotation of the rotational shaft 422A to offset the error.Alternatively, it is possible for the control system 185 to perform thisanalysis.

The position monitor 424A can be a very high resolution optical rotaryencoder that is built integrally with the rotational plate 423A. Theoptical rotary encoder uses optical sensing technology and on therotation of an internal code disc that has opaque lines and patterns onit. For example, the plate 423A is rotated (hence the name rotaryencoder) in a beam of light such as a light emitting diode and themarkings on the plate 423A act as shutters blocking and unblocking thelight. An internal photodiode detector senses the alternating light beamand the encoder's electronics convert the pattern into an electricalsignal that is then passed on to the control module 350 through theoutput of the encoder 424A.

In some implementations, the control module 350 can be designed with arapid internal dedicated controller solely for operating the rotarystepper motor 421A. For example, the rapid internal dedicated controllercan receive the high resolution position data from the encoder 424A andcan send a signal directly to the rotary stepper motor 421A to adjustthe position of the shaft 422A and thereby adjust the position of theprism 420.

Referring also to FIG. 4C, the illumination system 150 changes aspectral feature such as the bandwidth of the light beam 110A undercontrol of the control system 185, which interfaces with the controlmodule 350. For example, in order to coarsely and broadly control thebandwidth of the light beam 110A and the light beam 110, the controlmodule 350 sends a signal to the rotary stepper motor 421A of the rapidactuation system 420A to rotate the rotational shaft 422A from a firstangle θ1 (on the left side of FIG. 4C) to a second angle θ2 (whereΔθ=θ2−θ1) (on the right side of FIG. 4C). And, this change of angle ofthe shaft 422A is directly imparted to the plate 423A, which is fixed tothe shaft 422A, and thereby also imparted to the prism 420, which isfixed to the plate 423A. The rotation of the prism 420 from θ1 to θ2causes a corresponding change in the optical magnification OM 365 of thepulsed light beam 110A that interacts with the grating 400 from OM1 toOM2, and the change in the optical magnification 365 of the pulsed lightbeam 110A causes a change in the bandwidth of the pulsed light beam 110A(and the light beam 110 as well). The range of the bandwidth that can beachieved by rotating the prism 420 using this rapid actuation system420A can be a broad range and can be from about 100 femtometers (fm) toabout 450 fm. The overall bandwidth range achievable can be at least 250fm.

The rotation of the prism 420 associated with the rapid actuation system420A by one unit of rotation of the rotational shaft 422A causes thebandwidth of the pulsed light beam 110A to change by an amount that isless than a resolution of a bandwidth measurement device (for example,as a part of the metrology system 170, which is discussed below) thatmeasures the bandwidth of the pulsed light beam 110. The prism 420 canbe rotated by up to 15 degrees to achieve such a change in bandwidth. Inpractice, the amount of rotation of the prism 420 is constrained only bythe optical layout of the other components of the apparatus 430. Forexample, a rotation that is too large could cause the light beam 110A tobe displaced by an amount that is so large that the light beam 110A doesnot impinge upon the next prism 415. In some implementations, in orderto achieve a tuning of the bandwidth of the light beam 110A in anacceptable range, the prism 420 is capable of being rotated by 15degrees, without risk of the light beam 110A walking off any of theother prisms 405, 410, or 415. The prism 420 can be rotated by largerthan 15 degrees though it is not necessary with the current bandwidthrange requirements.

Referring again to FIG. 4A, the prism 410 can be mounted to an actuationsystem 410A that causes the prism 410 to rotate, and such rotation ofthe prism 410 can provide for fine control of the wavelength of thelight beam 110A. The actuation system 410A can include a rotary steppermotor that is controlled with a piezoelectric motor. The piezoelectricmotor operates by making use of the converse piezoelectric effect inwhich a material produces acoustic or ultrasonic vibrations in order toproduce a linear or rotary motion.

The next prism 415 that is closer to the grating 400, and has a sizethat is either larger than or equal to the size of the prism 420, can befixed in space in some implementations. The next prism 410 that iscloser to the grating 400 has a size that is either larger than or equalto the size of the prism 415.

The prism 405 that is closest to the grating 410 has a size that iseither larger than or equal to the size of the prism 410 (the prism 405is the largest prism of the beam expander). The prism 405 can be mountedto an actuation system 405A that causes the prism 405 to rotate and suchrotation of the prism 405 can provide for coarse control of thewavelength of the light beam 110A. For example, the prism 405 can berotated by 1-2 degrees to tune the wavelength of the light beam 110A(and thus the light beam 110) from about 193.2 nanometers (nm) to about193.5 nm. In some implementations, the actuation system 405A includes arotary stepper motor that includes a mounting surface (such as the plate423A) to which the prism 405 is fixed and a motor shaft that rotates themounting surface. The motor of the actuation system 405A can be apiezoelectric motor that is fifty times faster than a prior linearstepper motor and flexure combination design. Like the actuation system420A, the actuation system 405A can include an optical rotary encoderthat provides angular position feedback for the control system 185 orthe control module 350.

Referring to FIGS. 5A and 5B, in another implementation of the spectralfeature selection apparatus 530, a rapid actuation system 520A isdesigned to rotate R the prism 520 of a beam expander that is farthestfrom the grating 500 about the shaft axis AR.

The apparatus 530 includes an extending arm 525A that has a first region540A that is mechanically linked to the rotational plate 523A at thelocation of the shaft axis AR. The extending arm 525A has a secondregion 545A that is offset from the shaft axis AR along a direction inthe XY plane (and thus along a direction that is perpendicular to theshaft axis AR) so that the second region 545A is not intersected by theshaft axis AR. The prism 520 is mechanically linked to the second region545A.

Both the center of mass (the prism axis AP) of the prism 520 and theshaft axis AR remain parallel with the Z axis of the apparatus 530;however, the center of mass of the prism 520 is offset from the shaftaxis AR. A rotation of the extending arm 525A about the shaft axis AR byan angle Δθ imparts a combined movement to the prism 520: a rotation Rof the prism 520 about the shaft axis AR by an angle Δθ (see FIG. 5C)within the XY plane, and a linear translation T to the prism 520 along adirection that lies within the XY plane of the apparatus 530. In theexample of FIG. 5C, the prism 520 is rotated R from a first angle θ1 toa second angle θ2 and is translated T from a first position Pos1 in theXY plane to a second position Pos2 in the XY plane.

The linear translation T to the prism 520 thereby translates the lightbeam 110A along a direction that is parallel with the longer axis 501 ofthe surface 502 of the grating 500. The longer axis 501 also lies alongthe XY plane of the apparatus 530. By performing this translation of thelight beam 110A, it is possible to control which area or region of thegrating 500 is illuminated at the lower end of the range of possibleoptical magnifications OM. Moreover, the grating 500 and the surface 502of the grating is non-uniform; namely, some regions of the surface 502of the grating 500 impart a different change to the wavefront of thelight beam 110A than other regions of the surface 502 of the grating 500and some regions of the surface 502 impart more distortion to thewavefront of the light beam 110A than other regions of the surface 502.The control system 185 (or control module 350) can control the rapidactuation system 520A to thereby adjust the linear translation T to theprism 520 and adjust the translation of the light beam 110A along thelonger axis 501 to take advantage of the non-uniformity of the grating500 surface 502 and illuminate a higher distortion region of the gratingsurface 502 near one end of the grating surface 502 to raise thespectral bandwidth even more than the effect of simply lowering theoptical magnification would achieve.

Additionally, the linear translation T to the prism 520 also translatesthe hypotenuse H (see FIG. 5C) of the prism 520 during rotation of theprism 520 relative to the location of the light beam 110A. Thetranslation to the hypotenuse H therefore exposes new regions of thehypotenuse H to the light beam 110A during operation of the apparatus530. Over the lifetime of the apparatus 530, the prism 520 is rotatedfrom one end of its rotation range to the other end and also moreregions are exposed to the light beam 110A, which reduces the amount ofdamage imparted to the prism 520 by the light beam 110A.

Similar to the apparatus 430, the spectral feature selection apparatus530 also includes a grating 500, and the beam expander includes theprisms 505, 510, 515, which are positioned along the path of the lightbeam 110A between the prism 520 and the grating 500. The grating 500 andthe four prisms 505, 510, 515, 520 are configured to interact with thelight beam 110A produced by the optical source 105 after the light beam110A passes through an aperture 555 of the apparatus 530. The light beam110A travels along a path in the XY plane of the apparatus 530 from theaperture 555, through the prism 520, the prism 515, the prism 510, theprism 505, and then is reflected from the grating 500, and back throughthe consecutive prisms 505, 510, 515, 520 before exiting the apparatus530 through the aperture 555.

Referring to FIGS. 6A-6D, in other implementations, a rapid actuationsystem 620A is designed like the rapid actuation system 520A but with anadded secondary actuator 660A. The secondary actuator 660A is physicallycoupled to the prism 620 that is farthest from the grating 600. Thesecondary actuator 660A is configured to rotate the prism 620 about anaxis AH that lies in the XY plane and also lies in the plane of ahypotenuse H of the prism 620.

In some implementations, although not required the secondary actuator660A is controlled by the control module 350 (or the control system185). The secondary actuator 660A can be a manual screw and flexuredesign that is not controlled by the control module 350 or the controlsystem 185. For example, the actuator 660A could be set before thesystem 620A is used or periodically can be changed manually in betweenuses of the system 620A.

The prism 620 can therefore be rotated about the axis AH that lies inthe XY plane to enable greater control over where the light beam 110Aenters the prism 620 and the hypotenuse H of the prism 620 in order tobetter maintain the path of the light beam 110A through each of theprisms 615, 610, 605, and the grating 600. Specifically, rotation of theprism 620 about the axis AH enables the light beam 110A to be morefinely adjusted. For example, the prism 620 can be rotated about theaxis AH to ensure that the retro-reflected (that is, diffracted) lightbeam 110A from the grating 600 remains in the XY plane and is notdisplaced along the Z axis of the apparatus 630 even if the prism 620 isrotated about the AP or AR axis. It is beneficial to have this Z axisadjustment if the AP or AR axis is not perfectly aligned with the Zaxis. Additionally, it can be beneficial to rotate the prism 620 aboutthe AH axis because the extending arm 625A is a cantilever and it cansag or move in a manner along the Z axis such that it deflects about theaxis AH and the secondary actuator 660A can be used to offset thisdeflection.

Referring to FIG. 7 , in another implementation, a spectral featureselection apparatus 730 is designed with a dispersive optical element(such as a grating) 700, and a beam expander that includes three or morerefractive optical elements configured to optically magnify the lightbeam 110A as it travels from the aperture 755 toward the grating 700.The beam expander in this example includes four right-angled prisms 705,710, 715, 720 through which the pulsed light beam 110A is transmitted sothat the pulsed light beam 110A changes its optical magnification as itpasses through each right-angled prism, as discussed above. The prism705 closest to the grating 700 has a hypotenuse 705H that has thelargest length of those of the prisms of the beam expander. Eachconsecutive right-angled prism farther from the grating 700 than theprism 705 has a hypotenuse 710H, 715H, 720H that has a length that issmaller than or equal to the hypotenuse H of the adjacent right-angledprism that is closer to the grating 700.

The right-angled prism 705 that is closest to the grating 700 isarranged with its right angle α positioned away from the grating 700.This can be compared with the prism 405 of the apparatus 430 of FIG. 4A,in which its right angle is positioned toward or next to the grating400. Moreover, the region 707 between the right-angled prism 705 and thegrating 700 is void of any other optical element. There is no opticalelement (such as a reflective optical element or refractive opticalelement) between the prism 705 and the grating 700. Thus, the light beam110A travels between the prism 705 and the grating 700 without passingthrough any other optical element.

By flipping the prism 705 in this manner from the layout shown in FIG.4A to the layout shown in FIG. 7 , it is possible to obtain a greaterchange in optical magnification 365 of the light beam 110A for each unitof rotation of the prism 720, and thus enables more rapid adjustment ofthe bandwidth of the light beam 110A (and the light beam 110). In thisimplementation, in order to adjust the bandwidth of the light beam 110A,the prisms 720 and 710 are rotated in conjunction with each other toobtain a wide range of optical magnifications. Specifically, when theprism 720 and the prism 710 are rotated in conjunction with each other,the optical magnification can be adjusted from a lower value of 13× toan upper value of 75×, which is a wider range than possible in thelayout of FIG. 4A. The layout of the apparatus 730 provides the fastestway in which to adjust the bandwidth of the light beam 110A when the twoprisms 720, 710 are rotated in conjunction with each other. Theapparatus 730 has an overall different configuration than the apparatus430 of FIG. 4A and would require a redesign or reconfiguration of theother components (such as the optical source 105) of the illuminationsystem 150.

Next, a discussion about the other aspects of the photolithographysystem 100 is provided with reference to FIGS. 1, 8, and 9 .

As shown in FIG. 1 , the control system 185 is operatively connected tothe pulsed optical source 105 and to the spectral feature selectionapparatus 130. And, the scanner 115 includes a lithography controller140 operatively connected to the control system 185 and componentswithin the scanner 115.

The pulse repetition rate of the pulsed light beam 110 is the rate atwhich pulses of the light beam 110 are produced by the optical source105. Thus, for example, the repetition rate of the pulsed light beam 110is 1/Δt, where Δt is the time between the pulses. The control system 185is generally configured to control the repetition rate at which thepulsed light beam 110 is produced including modifying the repetitionrate of the pulsed light beam as it is exposing the wafer 120 in thescanner 115.

In some implementations, the scanner 115 triggers the optical source 105(through the communication between the controller 140 and the controlsystem 185) to produce the pulsed light beam 110, so the scanner 115controls the repetition rate, spectral features such as the bandwidth orwavelength, and/or the dose by way of the controller 140 and the controlsystem 185. For example, the controller 140 sends a signal to thecontrol system 185 to maintain the repetition rate of the light beam 110within a particular range of acceptable rates. The scanner 115 generallymaintains the repetition rate constant for each burst of pulses of thelight beam 110. A burst of pulses of the light beam 110 can correspondto an exposure field on the wafer 120. The exposure field is the area ofthe wafer 120 that is exposed in one scan of an exposure slit or windowwithin the scanner 115. A burst of pulses can include anywhere from 10to 500 pulses, for example.

The critical dimension (CD) is the smallest feature size that can beprinted on the wafer 120 by the system 100. The CD depends on thewavelength of the light beam 110. To maintain a uniform CD of themicroelectronic features printed on the wafer 120, and on other wafersexposed by the system 100, the center wavelength of the light beam 110should remain at an expected or target center wavelength or within arange of wavelengths around the target wavelength. Thus, in addition tomaintaining the center wavelength at the target center wavelength orwithin a range of acceptable wavelengths about the target centerwavelength, it is desirable to maintain the bandwidth of the light beam110 (the range of wavelengths in the light beam 110) to within anacceptable range of bandwidths.

In order to maintain the bandwidth of the light beam 110 to anacceptable range, or to adjust the bandwidth of the light beam 110, thecontrol system 185 is configured to determine an amount of adjustment tothe bandwidth of the pulsed light beam 110. Additionally, the controlsystem 185 is configured to send a signal to the spectral featureselection apparatus 130 to move at least one optical component of theapparatus 130 (for example, the prism 320) to thereby change thebandwidth of the pulsed light beam 110 by the determined adjustmentamount as the pulsed light beam 110 is exposing the wafer 120 to therebycompensate for the bandwidth variation caused by the modification of thepulse repetition rate of the pulsed light beam 110.

The bandwidth of the pulsed light beam 110 can be changed in between anytwo bursts of pulses. Moreover, the time that it takes for the bandwidthto be changed from a first value to a second value and also to stabilizeat the second value should be less than the time between the bursts ofpulses. For example, if the period of time between bursts is 50milliseconds (ms), then the total time to change the bandwidth from afirst value to a second value and stabilize at the second value shouldbe less than 50 ms. The control system 185 and the spectral featureselection apparatus 130 are designed to enable such a rapid change ofthe bandwidth, as discussed in detail below.

The controller 140 of the scanner 115 sends a signal to the controlsystem 185 to adjust or modify an aspect (such as the bandwidth or therepetition rate) of the pulsed light beam 110 that is being scannedacross the wafer 120. The signal sent to the control system 185 cancause the control system 185 to modify an electrical signal sent to thepulsed optical source 105 or an electrical signal sent to the apparatus130. For example, if the pulsed optical source 105 includes a gas laseramplifier then the electrical signal provides a pulsed current toelectrodes within one or more gas discharge chambers of the pulsedoptical source 105.

With reference again to FIG. 1 , the wafer 120 is placed on a wafertable constructed to hold the wafer 120 and connected to a positionerconfigured to accurately position the wafer 120 in accordance withcertain parameters and under control of the controller 140.

The photolithography system 100 can also include the metrology system170, which can include a sub-system that measures one or more spectralfeatures (such as the bandwidth or wavelength) of the light beam 110.Because of various disturbances applied to the photolithography system100 during operation, the value of the spectral feature (such as thebandwidth or the wavelength) of the light beam 110 at the wafer 120 maynot correspond to or match with the desired spectral feature (that is,the spectral feature that the scanner 115 expects). Thus, the spectralfeature (such as a characteristic bandwidth) of light beam 110 ismeasured or estimated during operation by estimating a value of a metricfrom the optical spectrum so that an operator or an automated system(for example, a feedback controller) can use the measured or estimatedbandwidth to adjust the properties of the optical source 105 and toadjust the optical spectrum of the light beam 110. The sub-system of themetrology system 170 measures the spectral feature (such as thebandwidth and/or the wavelength) of the light beam 110 based on thisoptical spectrum.

The metrology system 170 receives a portion of the light beam 110 thatis redirected from a beam separation device that is placed in a pathbetween the optical source 105 and the scanner 115. The beam separationdevice directs a first portion or percentage of the light beam 110 intothe metrology system 170 and directs a second portion or percentage ofthe light beam 110 toward the scanner 115. In some implementations, themajority of the light beam 110 is directed in the second portion towardthe scanner 115. For example, the beam separation device directs afraction (for example, 1-2%) of the light beam 110 into the metrologysystem 170. The beam separation device can be, for example, a beamsplitter.

The scanner 115 includes an optical arrangement having, for example, oneor more condenser lenses, a mask, and an objective arrangement. The maskis movable along one or more directions, such as along an optical axisof the light beam 110 or in a plane that is perpendicular to the opticalaxis. The objective arrangement includes a projection lens and enablesthe image transfer to occur from the mask to the photoresist on thewafer 120. The illuminator system adjusts the range of angles for thelight beam 110 impinging on the mask. The illuminator system alsohomogenizes (makes uniform) the intensity distribution of the light beam110 across the mask.

The scanner 115 can include, among other features, the lithographycontroller 140, air conditioning devices, and power supplies for variouselectrical components. In addition to controlling the repetition rate ofthe pulses of the light beam 110 (discussed above), the lithographycontroller 140 controls how layers are printed on the wafer 120. Thelithography controller 140 includes memory that stores information suchas process recipes and also may store information about which repetitionrates may be used or are preferable as described more fully below.

The wafer 120 is irradiated by the light beam 110. A process program orrecipe determines the length of the exposure on the wafer 120, the maskused, as well as other factors that affect the exposure. Duringlithography, as discussed above, a plurality of pulses of the light beam110 illuminates the same area of the wafer 120 to constitute anillumination dose. The number N of pulses of the light beam 110 thatilluminate the same area can be referred to as the exposure window orslit and the size of the slit can be controlled by an exposure slitplaced before the mask. In some implementations, the value of N is inthe tens, for example, from 10-100 pulses. In other implementations, thevalue of N is greater than 100 pulses, for example, from 100-500 pulses.

One or more of the mask, the objective arrangement, and the wafer 120can be moved relative to each other during the exposure to scan theexposure window across an exposure field. The exposure field is the areaof the wafer 120 that is exposed in one scan of the exposure slit orwindow.

Referring to FIG. 8 , an exemplary optical source 805 is a pulsed lasersource that produces a pulsed laser beam as the light beam 110. Theoptical source 805 is a two-stage laser system that includes a masteroscillator (MO) 800 that provides the seed light beam 110A to a poweramplifier (PA) 810. The master oscillator 800 typically includes a gainmedium in which amplification occurs and an optical feedback mechanismsuch as an optical resonator. The power amplifier 810 typically includesa gain medium in which amplification occurs when seeded with the seedlaser beam from the master oscillator 800. If the power amplifier 810 isdesigned as a regenerative ring resonator then it is described as apower ring amplifier (PRA) and in this case, enough optical feedback canbe provided from the ring design. The spectral feature selectionapparatus 130 receives the light beam 110A from the master oscillator800 to enable fine tuning of spectral parameters such as the centerwavelength and the bandwidth of the light beam 110A at relatively lowoutput pulse energies. The power amplifier 810 receives the light beam110A from the master oscillator 800 and amplifies this output to attainthe necessary power for output to use in photolithography.

The master oscillator 800 includes a discharge chamber having twoelongated electrodes, a laser gas that serves as the gain medium, and afan circulating the gas between the electrodes. A laser resonator isformed between the spectral feature selection apparatus 130 on one sideof the discharge chamber, and an output coupler 815 on a second side ofthe discharge chamber to output the seed light beam 110A to the poweramplifier 810.

The optical source 805 can also include a line center analysis module(LAM) 820 that receives an output from the output coupler 815, and oneor more beam modification optical systems 825 that modify the sizeand/or shape of the beam as needed. The line center analysis module 820is an example of one type of measurement system within the metrologysystem 170 that can be used to measure the wavelength (for example, thecenter wavelength) of the seed light beam.

The power amplifier 810 includes a power amplifier discharge chamber,and if it is a regenerative ring amplifier, the power amplifier alsoincludes a beam reflector or beam turning device 830 that reflects thelight beam back into the discharge chamber to form a circulating path.The power amplifier discharge chamber includes a pair of elongatedelectrodes, a laser gas that serves as the gain medium, and a fan forcirculating the gas between the electrodes. The seed light beam 110A isamplified by repeatedly passing through the power amplifier 810. Thebeam modification optical system 825 provides a way (for example, apartially-reflecting mirror) to in-couple the seed light beam 110A andto out-couple a portion of the amplified radiation from the poweramplifier to form the output light beam 110.

The laser gas used in the discharge chambers of the master oscillator800 and the power amplifier 810 can be any suitable gas for producing alaser beam around the required wavelengths and bandwidth. For example,the laser gas can be argon fluoride (ArF), which emits light at awavelength of about 193 nm, or krypton fluoride (KrF), which emits lightat a wavelength of about 248 nm.

The line center analysis module 820 monitors the wavelength of theoutput (the light beam 110A) of the master oscillator 800. The linecenter analysis module 820 can be placed at other locations within theoptical source 805, or it can be placed at the output of the opticalsource 805.

Referring to FIG. 9 , details about the control system 185 are providedthat relate to the aspects of the system and method described herein.The control system 185 can include other features not shown in FIG. 9 .In general, the control system 185 includes one or more of digitalelectronic circuitry, computer hardware, firmware, and software.

The control system 185 includes memory 900, which can be read-onlymemory and/or random access memory. Storage devices suitable fortangibly embodying computer program instructions and data include allforms of non-volatile memory, including, by way of example,semiconductor memory devices, such as EPROM, EEPROM, and flash memorydevices; magnetic disks such as internal hard disks and removable disks;magneto-optical disks; and CD-ROM disks. The control system 185 can alsoinclude one or more input devices 905 (such as a keyboard, touch screen,microphone, mouse, hand-held input device, etc.) and one or more outputdevices 910 (such as a speaker or a monitor).

The control system 185 includes one or more programmable processors 915,and one or more computer program products 920 tangibly embodied in amachine-readable storage device for execution by a programmableprocessor (such as the processors 915). The one or more programmableprocessors 915 can each execute a program of instructions to performdesired functions by operating on input data and generating appropriateoutput. Generally, the processor 915 receives instructions and data frommemory 900. Any of the foregoing may be supplemented by, or incorporatedin, specially designed ASICs (application-specific integrated circuits).

The control system 185 includes, among other components, a spectralfeature analysis module 925, a lithography analysis module 930, adecision module 935, a light source actuation module 950, a lithographyactuation module 955, and a beam preparation actuation module 960. Eachof these modules can be a set of computer program products executed byone or more processors such as the processors 915. Moreover, any of themodules 925, 930, 935, 950, 955, 960 can access data stored within thememory 900.

The spectral feature analysis module 925 receives the output from themetrology system 170. The lithography analysis module 930 receivesinformation from the lithography controller 140 of the scanner 115. Thedecision module 935 receives the outputs from the analyses modules (suchas the modules 925 and 930) and determines which actuation module ormodules need to be activated based on the outputs from the analysesmodules. The light source actuation module 950 is connected to one ormore of the optical source 105 and the spectral feature selectionapparatus 130. The lithography actuation module 955 is connected to thescanner 115, and specifically to the lithography controller 140. Thebeam preparation actuation module 960 is connected to one or morecomponents of the beam preparation system 112.

While only a few modules are shown in FIG. 9 , it is possible for thecontrol system 185 to include other modules. Additionally, although thecontrol system 185 is represented as a box in which all of thecomponents appear to be co-located, it is possible for the controlsystem 185 to be made up of components that are physically remote fromeach other. For example, the light source actuation module 950 can bephysically co-located with the optical source 105 or the spectralfeature selection apparatus 130.

In general, the control system 185 receives at least some informationabout the light beam 110 from the metrology system 170, and the spectralfeature analysis module 925 performs an analysis on the information todetermine how to adjust one or more spectral features (for example, thebandwidth) of the light beam 110 supplied to the scanner 115. Based onthis determination, the control system 185 sends signals to the spectralfeature selection apparatus 130 and/or the optical source 105 to controloperation of the optical source 105 via the control module 350.

In general, the spectral feature analysis module 925 performs all of theanalysis needed to estimate one or more spectral features (for example,the wavelength and/or the bandwidth) of the light beam 110. The outputof the spectral feature analysis module 925 is an estimated value of thespectral feature.

The spectral feature analysis module 925 includes a comparison blockconnected to receive the estimated spectral feature and also connectedto receive a spectral feature target value. In general, the comparisonblock outputs a spectral feature error value that represents adifference between the spectral feature target value and the estimatedvalue. The decision module 935 receives the spectral feature error valueand determines how best to effect a correction to the system 100 inorder to adjust the spectral feature. Thus, the decision module 935sends a signal to the light source actuation module 950, whichdetermines how to adjust the spectral feature selection apparatus 130(or the optical source 105) based on the spectral feature error value.The output of the light source actuation module 950 includes a set ofactuator commands that are sent to the spectral feature selectionapparatus 130. For example, light source actuation module 950 sends thecommands to the control module 350, which is connected to the actuationsystems within the apparatus 330.

The control system 185 causes the optical source 105 to operate at agiven repetition rate. More specifically, the scanner 115 sends atrigger signal to the optical source 105 for every pulse (that is, on apulse-to-pulse basis) and the time interval between those triggersignals can be arbitrary, but when the scanner 115 sends trigger signalsat regular intervals then the rate of those signals is a repetitionrate. The repetition rate can be a rate requested by the scanner 115.

The repetition rate of the pulses produced by the power amplifier 810 isdetermined by the repetition rate at which the master oscillator 800 iscontrolled by the control system 185, under the instructions from thecontroller 140 in the scanner 115. The repetition rate of the pulsesoutput from the power amplifier 810 is the repetition rate seen by thescanner 115.

The prism 320 (or prism 420, 520, 620, 720) can be used for coarse,large range, slow bandwidth control. By contrast, the bandwidth can becontrolled in a fine and narrow range and even more rapidly bycontrolling a differential timing between the activation of theelectrodes within the MO 800 and the PRA 810.

Other implementations are within the scope of the following claims.

For example, in other implementations, the prism 315 is mounted to itsown actuation system 315A that causes the prism 315 to rotate, and suchrotation changes the angle of incidence of the light beam 110A impingingupon the grating 300 and can be used to provide for fine control of thewavelength of the light beam 110A. The actuation system 315A can includea piezoelectric rotation stage as an actuator. In these otherimplementations, the prism 310 can be mounted to an actuation system310A that provides for fine control of the bandwidth of the light beam110A. Such an actuation system 310A can include as an actuator a steppermotor rotary stage.

What is claimed is:
 1. A spectral feature selection apparatuscomprising: a dispersive optical element; a beam expander including aplurality of prisms arranged in a path between the dispersive opticalelement and an aperture; and at least one actuation system comprising arapid actuator including a rotatable shaft to which a prism in the beamexpander is fixed; wherein: the dispersive optical element and the beamexpander are arranged such that a light beam interacts with theaperture, the beam expander, and the dispersive optical element along anoptical path that lies in an XY plane of the apparatus; the rotatableshaft is configured to rotate about a shaft axis that is perpendicularto the XY plane and thereby causing the prism to rotate about a prismaxis that is parallel with the shaft axis; and the rapid actuator lacksmechanical memory and lacks an energy ground state.
 2. The spectralfeature selection apparatus of claim 1, further comprising a controlsystem connected to the actuation system, and configured to send asignal to the actuation system instructing the rapid actuator to rotatethe rotatable shaft to thereby rotate the prism fixed to the rotatableshaft.
 3. The spectral feature selection apparatus of claim 1, whereinthe rapid actuator is associated with the prism that is farthest fromthe dispersive optical element and is the smallest prism within the beamexpander.
 4. The spectral feature selection apparatus of claim 1,wherein the rapid actuator is configured to move the prism fixed to therotatable shaft by 15 degrees in less than 50 milliseconds.
 5. Thespectral feature selection apparatus of claim 1, wherein a center ofmass of the prism fixed to the rotatable shaft is aligned with the shaftaxis and the prism is fixed to the rotatable shaft such that it isdirectly rotated about its rotation axis as the rotatable shaft isrotated about its shaft axis.
 6. The spectral feature selectionapparatus of claim 1, wherein a center of mass of the prism fixed to therotatable shaft is offset from the shaft axis.
 7. The spectral featureselection apparatus of claim 6, wherein rotatable shaft is fixed to afirst end of an arm and the prism fixed to the rotatable shaft is fixedto a second end of the arm.
 8. The spectral feature selection apparatusof claim 7, wherein the prism fixed to the rotatable shaft is configuredto be rotated about its center of mass and about the shaft axis.
 9. Thespectral feature selection apparatus of claim 1, wherein a rotationalstep of the rotatable shaft correlates to a rotational step of the prismfixed to the rotatable shaft, and the prism rotates by an incrementalunit for each unit rotation of the rotatable shaft such that the prismrotates as fast as the rotational shaft.
 10. The spectral featureselection apparatus of claim 1, wherein the actuation system includes amotor controller configured to control a rotation of the rotatable shaftusing variable-frequency drive control.
 11. The spectral featureselection apparatus of claim 1, wherein the dispersive optical elementand the beam expander are arranged in a Littrow configuration.
 12. Thespectral feature selection apparatus of claim 1, wherein the rotatableshaft is configured to rotate about the shaft axis by 360°.
 13. Thespectral feature selection apparatus of claim 1, wherein each locationof the rotatable shaft is at the same energy as each of the otherlocations of the rotatable shaft, and the rotatable shaft lacks apreferred resting location with a low potential energy.
 14. Anillumination system comprising: an optical source configured to producea light beam; and a spectral feature selection apparatus arranged tointeract with the light beam produced by the optical source, thespectral feature selection apparatus comprising: a dispersive opticalelement; a beam expander including a plurality of prisms arranged in apath between the dispersive optical element and an aperture throughwhich the light beam of the optical source can pass; and at least oneactuation system comprising a rapid actuator including a rotatable shaftto which a prism in the beam expander is fixed; wherein: the dispersiveoptical element and the beam expander are arranged such that the lightbeam of the optical source interacts with the aperture, the prisms, andthe dispersive optical element along an optical path that lies in an XYplane of the apparatus; the rotatable shaft is configured to rotateabout a shaft axis that is perpendicular to the XY plane and therebycausing the prism to rotate about a prism axis that is parallel with theshaft axis; and the rapid actuator lacks mechanical memory and lacks anenergy ground state.
 15. The illumination system of claim 14, furthercomprising a control module in communication with the spectral featureselection apparatus, the control module configured to change a bandwidthof the light beam by sending a signal to the actuation system to therebyrotate the rotatable shaft of the rapid actuator and the prism connectedto the rapid actuator.
 16. The illumination system of claim 15, whereinthe rapid actuator is configured to change the bandwidth of the lightbeam from a first target value to a second target value and stabilizethe bandwidth at the second target value in less than 50 milliseconds(ms).
 17. The illumination system of claim 15, wherein the rapidactuator is configured to change the bandwidth of the light beam from afirst target value to a second target value and stabilize the bandwidthat the second target value in a time that is less than a time betweenbursts of pulses of the light beam produced by the optical source. 18.The illumination system of claim 14, further comprising a measurementdevice configured to measure a bandwidth of the light beam output fromthe optical source, wherein rotation of the prism connected to the rapidactuator by a unit of rotation of the rotatable shaft causes a bandwidthof the light beam to change by an amount that is less than a resolutionof the measurement device.
 19. The illumination system of claim 14,wherein the rapid actuator includes a piezoelectric motor.
 20. Theillumination system of claim 14, wherein the optical source is atwo-stage pulsed laser source that includes a master oscillatorproviding a seed light beam to a power amplifier, and the spectralfeature selection apparatus receives the produced light beam from themaster oscillator.